Step 1: Make the dressing. Place the 1 tablespoon of lemon juice and the wine vinegar, mustard, and garlic in a mixing bowl. Season with salt and pepper to taste and whisk until the salt dissolves. Whisk in the 1/4 cup of olive oil in a thin stream, then whisk in the anchovies, if using, capers, and basil. Taste for seasoning, adding more vinegar and/or salt as necessary; the dressing should be highly seasoned. Set the dressing aside.

Step 2: Make the salad. Place the tuna in a shallow dish with 2 tablespoons of the olive oil and the 2 tablespoons of lemon juice. Season with sea salt and cracked black peppercorns to taste. Turn the fish a couple of times to coat thoroughly and let marinate, in the refrigerator, covered, for 30 minutes.

Step 3: Place the potatoes in a medium saucepan and add lightly salted water to cover. Bring the potatoes to a boil, reduce the heat, and let simmer until just tender, 8 to 10 minutes. Using a slotted spoon, transfer the potatoes to a colander, run cold water over them, and let them drain. Let the water in the saucepan return to a boil. Add the haricots verts and let cook in the rapidly boiling water until they are tender-crisp, about 1 minute. Drain the beans, run cold water over them, then drain again.

Step 4: Set up the grill for direct grilling and preheat to high.

Step 5: When ready to cook, if using a vegetable grate, preheat it for 5 minutes. Toss the potatoes, green beans, and onion wedges with the remaining 2 tablespoons of olive oil and season with salt and pepper to taste. Arrange the vegetables on the hot vegetable grate or grill grate and grill, turning with tongs, until nicely charred on the outside, 3 to 5 minutes per side. Transfer to a platter and let cool.

Step 6: Brush and oil the grill grate. Drain the tuna steaks and place them in the basket or on the hot grate. Grill the tuna until cooked to taste, 3 to 5 minutes per side for medium-rare. Transfer the tuna to a cutting board, let it rest for 3 minutes, then cut it into thin crosswise slices.

Step 7: Just before serving: mound the mesclun in the center of a large platter and arrange the tomato wedges, haricots verts, potatoes, and onion wedges (cut off the root end), hard-cooked eggs, and olives in an attractive pattern around the greens. Whisk the dressing again, then spoon it over the salad. Garnish the salad with the basil sprigs and serve.

Tips:

If you aren’t using a vegetable grate, the easiest way to grill the haricots verts is to skewer them side by side together; that way they won’t fall through the grill grate.

The salad can be prepared ahead up to this point, unless you wish to serve the tuna hot. in that case, grill it at the last minute.